JOEL - It was not considered a "sin" to partake of the fruit; it was a "transgression". There is a big difference. To commit a sin one must understand right from wrong and intentionally choose to do the wrong. Adam and Eve knew fully well the plan God had for His children on the earth. There was no veil of forgetfulness over their minds about the pre-earth life and what was decided there. Even though in their inocent state they did not comprehend good and evil, they did have this knowledge of God's plan of salvation. Because of this knowledge and because God gave them their agency to choose for themselves they decided to choose the greater of the two commands God gave them.
1. If they wanted to stay in the Garden forever God commanded them to not partake of the fruit of the tree.
2. God also gave them a greater commandment; that they must multiply and replenish the earth; something they could not do if they remained in the garden.
They made the correct choice to do the latter. Not chosing to do the first one was not a sin.
Satan's intention was not to help with God's plan; he simply did something that God knew he would do; tempt His children to disobey a command; for this he was cursed and cast out.
In thinking about all this we really don't know for sure how literal to take the descriptions of events and characters and what they did in the Garden of Eden. The Bible may be speaking in figurative terms in its description of much of this.